#include <stdatomic.h>
#include <pthread.h>
#include <assert.h>
#include <stdbool.h>

atomic_bool flag[2];
atomic_int turn;

int other( int x ) { return x == 0 ? 1 : 0; }

atomic_int critical;

void *worker( void *_tid ) {
    int tid = (int)_tid;
    flag[ tid ] = true;
    turn = other( tid );
    while ( flag[ other( tid ) ] && turn == other( tid ) ) { }
    // critical start
    atomic_fetch_add_explicit( &critical, 1, memory_order_relaxed );
    atomic_fetch_sub_explicit( &critical, 1, memory_order_relaxed );
    // end
    flag[ tid ] = false;
    return 0;
}

int main() {
    pthread_t t1, t2;
    pthread_create( &t1, 0, worker, (void *)0 );
    pthread_create( &t2, 0, worker, (void *)1 );
    while ( true )
        assert( atomic_load_explicit( &critical, memory_order_relaxed ) <= 1 );
    pthread_join( t1, 0 );
    pthread_join( t2, 0 );
    return 0;
}